1. Field of the Invention
The present invention relates to a communication method, a communication system and communication device for performing the communication method, and a program for controlling the communication device.
2. Description of the Related Art
There are various types of networks for enabling communication between a plurality of terminals. Networks using Internet protocol (IP) as a transmission protocol (hereinafter referred to as “IP networks”) are well known.
In IP networks, mechanisms that provide media communication, such as audio and video communication, between terminals, e.g., IP telephony and videoconferencing systems, have been developed. Recently, the establishment of enterprise networks has attracted attention.
In media communication, such as audio and video communication, for example, terminals connected to an IP network perform call control operations, such as setup, modification, teardown, and completion response of a media communication session. Session initiation protocol (SIP) has attracted attention as a session control protocol. SIP is an application layer protocol for realizing, for example, telephone call setup on an IP network, and is standardized by the Internet Engineering Task Force (IETF). SIP is used for transmission or reception of IP address information necessary for setting up a session in the application layer or real-time transport protocol (RTP) IP address information to be used by transmitting and receiving terminals. RTP is a real-time data transfer protocol for use in communication services, such as Voice over IP (VoIP) services for transmitting audio via IP packets or video transmission services, and is standardized by the IETF.
Recently, SIP, which is a call control protocol, has been widely used, mainly, in VoIP (Internet telephony) services, and service infrastructures centered on SIP servers have been established. At present, instant messaging (IM) services are often implemented using SIP. However, SIP does not support actual data transfer (also referred to as “file transfer”), such as transfer of audio files or image files, between terminals. Some methods for enabling file transfer by using a combination of standard protocols and original protocols, other than SIP, have been proposed (see Japanese Unexamined Patent Application Publication No. 2004-147128 and No. 2005-051445).